High-metallicity effects in BVRI colour-magnitude diagrams : the globular cluster NGC 6553

Abstract

We present BVRI CCD-photometry of about 15000 stars in the possibly super-metal-rich globular cluster NGC 6553. The photometry extends to V = 21.5 reaching the cluster turn-off region. In the V vs. (V-I) diagram the red giant branch forms an arc, where the red tip is as faint as the horizontal branch, due to blanketing effects which are particularly strong in the highmetallicity late-type stars Depending on the amount ofblanketing in the spectral region and the combination of filters, the tip of the giant branch turns back towards the blue in certain colours like (B - V). Such distributions in the colour-magnitude diagrams (CMD) might be misleadingly interpreted in terms of a scatter in metallicity. The horizontal branch (HB) is so extremely red that it partially overlaps the red giant branch (RGB). The shape of the RGB and the relative positions ofthe RGB and the HB differ significantly from those of globular clusters usually called metal-rich as, for example, 47 Tuc. Using as age criterion the magnitude difference between the turn-off and the HB, we have estimated that the age of NGC 6553 should be slightly below that of classical globular clusters.